Mixed office waste is receiving considerable interest as a source of recycled fiber, particularly in printing and writing grades. Currently, it is one of the major grades of waste paper available accounting for almost 3,000,000 tons/year. The major problem in using mixed office waste, and the reason why it is currently under-utilized, is that it contains a high percentage of difficult to deink, non-impact printed material.
Xerography is a method for producing documents by an electrostatic technique. Xerography is used by both xerographic copying machines (photostatic duplicating machines) and by "laser" printers to produce copies or computer drafted originals, respectively. Such xerographic machines are now widely used as office equipment, and generate a great amount of waste paper. This waste paper is a large source of high-quality recycled fiber.
In xerography, an electrostatic image is produced on an electrostatically charged plate (photoconductive insulating surface) by a radiation differential, i.e., different amounts of radiation falling on different parts of the plate. A dark powder, generally referred to as a toner, of opposite charge to that of the plate, is applied and adheres to the charged areas, from which it is transferred to paper or a similar medium. Xerography is thus a reproduction method that employs a dry or low moisture (xeric) "ink".
When one document is being produced from another fixed-medium document, such as in xerographic copying, ultraviolet light is passed through the document to be copied. Where the ultraviolet light is not blocked, it discharges the plate; where the ultraviolet light is blocked by inked sections or the like, the plate is not discharged. A similar xerographic technique is employed by laser printers.
The toner is generally comprised of carbon black and certain thermoplastic polymers (i.e., binders), with other ingredients. The toner is applied to a paper sheet by the non-impact (i.e., without localized impact) electrostatic method, and fused by heat to promote adhesion of the thermoplastic binder to the sheet.
Polymeric thermoplastic binders found in xerographic toners generally comprise from about 80 to about 90 weight percent thereof. Such thermoplastic binders routinely have softening temperatures in the range from about 60.degree. C. to about 100.degree. C. which is the common temperature range for adhesion fusion of such toners to the paper substrate. Some toners employ thermoplastic binders of substantially a single type of polymer, such as a random styrene-predominant copolymer of high-carbon alkyl acrylate ester, for instance 2-ethylhexyl acrylate. Other binders include a styrenevinylcarboxylic acid ester, a styrene-butadiene copolymer, and a bis-phenol A resin and ester. Still other toners have thermoplastic binders that are polymer blends, for instance blends of poly(methyl methacrylate) and nylon.
Waste paper (i.e., commonly referred to in the art and herein as "xerographic paper" or "xerographic waste paper") produced by xerography is a significant potential source of recycled fiber. The paper of xerographic paper is routinely manufactured from a bleached chemical pulp which is a higher grade of pulp than for instance that used for newspaper. Paper recycling normally requires the fiber to be repulped and deinked. Conventional deinking processes, however, were developed for the high volume recycling of newspaper. Xerographic toners are different from the newsprint inks for which deinking processes were designed. The conventional deinking processes used for recycling newspapers do not achieve the performance level on xerographic paper required for a high grade of recycled paper.
Conventional deinking (such as that used for newsprint deinking) is comprised of three distinct steps. First the ink is removed, or separated, from the paper fiber. This usually occurs while the waste paper is being repulped in water and is routinely aided by caustic and surfactant. Secondly, the separated ink is dispersed to a small particle size. The dispersion step usually occurs concomitantly with the separation step. In the third step, the dispersed ink is usually separated from the repulped fiber slurry by washing or flotation.
Efficient deinking demands both a successful separation of ink from the fiber and a successful separation of the ink dispersion from the fiber slurry. A deinking process that successfully separates the ink from the fiber and successfully disperses the ink into the aqueous phase of the slurry as small particles will be inadequate if thereafter it does not provide effective removal of the ink from the fiber slurry. Dispersed ink particles that are carried along with the fiber in the slurry will in some degree be retained on the fiber during paper formation, resulting in a general gray hue or distinct spots, and commonly a low brightness.
The performance of a deinking process is usually judged by a dirt or speck count and by total reflectance, i.e., brightness, of the paper product produced from the recycled, deinked pulp. In a speck or dirt count method, a test sheet is formed of the repulped and deinked slurry, and then the number of ink particles that can be seen on the sheet by the naked eye per unit weight or unit area are counted. The ink or toner particles that can be detected, and thus counted, by this method are only those having a diameter greater than about 50 micrometers. However, most particles that survive a deinking process are smaller than 50 micrometers and although they cannot be seen by the naked eye, they lower the brightness more than the larger particles. The presence of such deleterious smaller particles are thus detectable from the reflectance, or brightness, level of the test sheet.
Simple alkaline repulping methods have been found to separate xerographic toners from fiber. Such repulping disperses the toner to a wide distribution of particle sizes, including small, intermediate and large (above &gt;50 micrometers) particle sizes. The separation of the intermediate and large ink particles cannot be achieved by simple washing. Washing requires small ink particles (about &lt;10 micrometers) that can pass through the fiber mat and washer wire. Most North American recycle mills were designed to handle pulps such as repulped newsprint that respond to simple washing, and these washing systems do not sufficiently separate the intermediate and large xerographic toner particles from the slurry. The other conventional separation methods require particles that are larger than the small and intermediate size xerographic paper toner particles.
Flotation for instance requires large particles that can be induced to adhere to air bubbles. Newer recycle mills have both washing and flotation capability, and the combination of such methods may produce a relatively clean pulp, but with a reduced pulp yield believed to be caused by low selectivity in the flotation step.
Screening requires that the particles be larger in all dimensions than the screen openings and have a high enough modulus of elasticity or plasticity to avoid being extruded into the screen openings. Even the intermediate size xerographic ink particles may be small enough in their smallest dimension to pass through such openings.
Centrifugal cleaners require an appropriate particle size, particle shape and a density differential for removal. The intended reject particles (i.e., ink particles) must differ in density from the intended "accepts" (i.e., the fiber slurry). It is believed that the irregular flat shape of xerographic ink particles as repulped creates at least some of the difficulties encountered in separation by centrifugal cleaners.
Co-pending U.S. patent application Ser. No. 07/942,816 (Cosper), filed on Sep. 10, 1992, discloses a unique method whereby certain low molecular weight, water-insoluble organic esters act to agglomerate toner particles during the repulping process. By agglomerating toner fragments they can be separated from the fiber slurry by centrifugal cleaners and screens. This particular method for deinking of xerographic paper comprises repulping the xerographic paper to separate the xerographic toner from the fiber, adding a substantially water insoluble organic ester to the repulped slurry, whereby the xerographic toner particles within the slurry are at least partially agglomerated to larger particle sizes, and then removing the xerographic toner particles by one or more liquid/solid separation techniques. The ester typically has the structure of Formula I below: EQU C.sub.m H.sub.n (COOR).sub.x (I)
wherein x is an integer of at least 1, R is an alkyl having from 1 to about 12 carbon atoms, m is an integer having a value of from about 1 to about 20, or 22, and n is an integer having a value of from about m-2 to about 2m+1, and when x is more than 1, each R is separately an alkyl having from 1 to about 12 carbon atoms; provided that the ester has at least about eight total carbons. The ester is preferably diisobutyl succinate, diisobutyl glutarate, diisobutyl adipate, trans-methyl cinnamate or combinations thereof.
The present inventors have discovered that some toners do not respond well to ester treatment. That is, little or no aggregation occurs even at high doses of the agglomerating agent. In other instances, toners can be readily overdosed to afford large, tacky particles which are unsuitable for mechanical separation.
The present inventors have undertaken the task of exploring chemistries capable of enhancing the performance of the organic ester agglomerating agent in the repulping of xerographic waste paper. Through extensive experimentation, the present inventors have developed a universal treatment system capable of causing toner agglomeration in most xerographic waste paper. The novel treatment system according to the present invention involves the addition of an organic ester together with a water insoluble, polymeric particulate material to enhance the agglomeration of xerographic toners during waste paper repulping.
The combined use of liquids and solid particulates to agglomerate toners has been disclosed in U.S. Pat. Nos. 4,276,118 (Quick), which issued on Jun. 30, 1981, and 4,820,379 (Darlington), which issued on Apr. 11, 1989. The Quick patent discloses a process which uses solid polymers such as polystyrene or poly(vinyl chloride) (PVC) in combination with a water-insoluble liquid which must be an aliphatic alcohol or a saturated aliphatic hydrocarbon. In accordance with Quick the temperature of the pulp slurry should be at or above the softening point of the toner.
The present inventors have found that aliphatic alcohols alone do not agglomerate toners, at least at ambient temperature. The process of the Quick patent has also experienced problems in the separation of agglomerates from pulp.
To the contrary, the present invention is capable of agglomerating toners at very low temperatures, e.g., 25.degree. C., while the Quick patent requires elevated temperatures. This is because the liquid esters of the present invention tackify toner without heat; whereas the aliphatic alcohol or hydrocarbon according to the Quick patent does not. Moreover, the agglomerating agents of the present invention are capable of deinking mixed office waste that contains xerographic as well as impact printed paper; whereas the treatment process according to the Quick patent had difficulty removing toner when repulping mixed office waste.
The Darlington patent discloses a two component treatment system comprising an aqueous medium containing polymeric material (e.g., polystyrene or a styrenecarboxylic acid copolymer) and a compound or mixture of compounds of the Formula R(OCH.sub.2 CH.sub.2)nR.sup.1 wherein R is a C.sub.6 to C.sub.20 linear or branched alkyl, and R.sup.1 is a halogen or the like. These agglomerating agents require high process temperatures, i.e., 60.degree.-75.degree. C. and very specific agglomerating chemicals. Halogen-terminated polyethers are not common materials of commerce resulting in periodic cost and supply problems.
The present invention also provides many additional advantages which shall become apparent as described below.